The present invention relates to membranes for use in molecular sieving, and more particularly to inorganic membranes providing high sieving flux and selectivity.
Membrane-based separations are energy efficient and cost effective. They represent promising alternatives to energy-intensive distillation, cryogenic separation, or pressure swing adsorption in applications such as purification of sub-quality natural gas, air separation, removal of VOCs and NOx, and hydrogen recovery from processing gases and feed stocks. Microporous inorganic membranes have attracted considerable attention for gas separation due to their excellent thermal and chemical stability, good erosion resistance and high pressure stability compared to conventional polymeric membranes (e.g. cellulusic derivative, polysulfone, polyamide, or polyimide membrane).
An inorganic membrane system generally consists of a macroporous support providing mechanical strength for an overlying thin, either dense or porous, separation membrane. Dense membranes prepared from palladium or perovskite only allow certain gases (such as H2 or O2) to transport via mechanisms such as solution-diffusion or solid-state ionic conduction. Such membranes require high capital investment due to the use of precious metals and/or extreme synthesis conditions. In contrast, porous silica membranes with tunable pore sizes can be processed by a simple dip-coating or spin-coating procedure and can be used potentially in a large variety of gas separations. Microporous silica membranes have been demonstrated to show promising molecular sieving characteristics.
Certain techniques have been developed to process porous silica membranes. They include sol-gel synthesis, leaching, and chemical vapor deposition. Among these, sol-gel processing attracts the most attention do to its excellent processibility and its potential to precisely control pore size and pore structure. Strategies for the fundamental physical and chemical phenomena involved in the deposition of colloidal ceramic dispersions (sols) on porous supports for precise pore size and porosity control have been proposed and discussed in Brinker et al., xe2x80x9cSol-gel Strategies for Controlled Porosity Inorganic Materials,xe2x80x9d J. Membr. Sci., 94 (1995) 85, incorporated herein by reference. Three keys to membrane production are 1) avoidance of cracks, pinholes or other defects that would reduce the selectivity; 2) precise pore size control (0.3-0.4 nm in diameter) so that separation occurs on the basis of size by molecular filtration or xe2x80x9csievingxe2x80x9d; and 3) maximization of the volume fraction porosity and minimization of the membrane thickness to maximize flux.
Forming a microporous silica membrane on top of a home-made disk-shaped, double-coated xcex3-alumina support has been described in the article De Vos et al., xe2x80x9cImproved Performance of Silica Membranes for as Separation,xe2x80x9d J Membr. Sci., 143 (1-2) (1998) 37-51, incorporated herein by reference. The coating and calcination process was repeated once to minimize potential defects. Both silica sol preparation and membrane processing were similar to those developed by De Lange, Hekdrink and Keizer, described in De Lange et al., xe2x80x9cFormation and Characterization of Supported Microporous Ceramic Membranes Prepared by So-Gel Modification Techniques,xe2x80x9d J. Membr. Sci., 99 (1995) 57-75, incorporated herein by reference. The improvement of membrane performance was attributed to the processing under class-10 clean room conditions, which increases the cost of manufacturing. Prior art membranes have employed repeated-coating processes to reduce intrinsic defects. The multi-step coating process results in reduction of the number of defect sites, thus increasing selectivity but at the expense of permeation flux and cost of manufacturing. Although high selectivity can significantly reduce either feed loss (single stage) or recompression costs (multiple stage), high permeation flux is necessary to achieve commercially satisfactory production rates. Therefore, the deadlock of tradeoff between selectivity and flux needs to be overcome.
The present invention provides for an inexpensive supported membrane capable of molecular sieving.
The present invention further provides for a uniform intermediate layer on a substrate to allow the deposition of a second, top microporous layer which is relatively defect free and a method for producing the defect free layer.
The present invention also provides for a supported membrane with precise pore size to achieve molecular sieving with maximized flux and selectivity.
There is described a dual-layer inorganic microporous membrane capable of molecular sieving, and methods for production of such membranes. The inorganic microporous supported membrane includes a porous substrate which supports a first inorganic porous membrane having an average pore size of less than about 25 xc3x85 and a second inorganic porous membrane coating the first inorganic membrane having an average pore size of less than about 6 xc3x85.
The dual-layered membrane is produced by contacting the porous membrane support with a surfactant-containing inorganic polymeric sol, resulting in a surfactant/inorganic polymer coated membrane support. The surfactant/inorganic polymer coated membrane support is dried, producing a self-assembled surfactant-templated surfactant/inorganic polymer composite film. This supported composite film is calcined to remove the surfactant templates to produce a surfactant-templated micro- or mesoporous membrane substrate, which serves as an intermediate layer surfactant-templated membrane. The intermediate layer surfactant-templated membrane is then contacted with a second inorganic polymeric sol producing an inorganic polymeric sol coated substrate which is dried, producing a supported inorganic polymer coated dual layer structure. This supported dual-layered structure is then calcined to produce a dual-layered microporous supported membrane in accordance with the present invention.
In one embodiment, both of the polymeric sols include silica, aligomers or polymers. The first or intermediate layer of the dual-layer supported membrane generally has a thickness of less than 100 nm and the second or top layer has a thickness of less than about 100 xc3x85. The average pore diameter of the dual-layer supported membrane gradually decreases in size from about 40 to 60 xc3x85 for the support, to about 10 to 25 xc3x85 for the intermediate layer, to about 2 to 5 xc3x85 for the top microporous layer.
The calcining procedure of the second, top layer includes calcining under a vacuum of less than about 4 psia at a temperature of between 200 to 400xc2x0 C. and further calcining at a temperature of between 300 to 600xc2x0 C. The calcining of the first or intermediate layer involves calcining at a temperature between 100 to 150xc2x0 C. and further heating between 500 to 600xc2x0 C. The drying of the sols involves drying under conditions of low relative pressure of the liquid constituents.